The team, drawn from the Colorado School of Mines and the California-based company Mango Materials, tested a hollow fiber membrane contactor, a device in which thousands of thin polypropylene tubes act as selective barriers. Liquid water flows through the inside of the fibers while gas flows around them, and because the hydrophobic membrane repels liquid water but lets gas molecules pass, oxygen can dissolve into the liquid without a single bubble ever forming. Unlike sparging, the interfacial area of such a membrane is fixed and independent of flow rates, and the liquid and gas streams can be tuned independently to thin the boundary layers that limit transfer. The driving force is simply the difference in gas concentration, or partial pressure, between the gas and liquid sides, which is why the researchers suspected gravity should play no role at all. Proving it, however, required taking the apparatus somewhere gravity itself could be switched off.

Their motivation goes beyond fundamental fluid physics. Mango Materials has developed a membrane-aerated bioreactor in which methanotrophic bacteria consume methane and oxygen to build biomass and, when nutrients are withheld, the biopolymer poly-3-hydroxybutyrate, or PHB, a versatile plastic used in additive manufacturing, textiles and even food additives. Methane for such a reactor could be harvested from the Sabatier reaction aboard the International Space Station, where it is currently vented as waste, or pulled directly from the Martian atmosphere. Converting that carbon into PHB would be a textbook example of in-situ resource utilization. But before a membrane-aerated bioreactor can fly, engineers must be sure the gas delivery step itself is indifferent to gravity, so that any gravitational effects seen later in a full bioreactor can be confidently attributed to biology rather than plumbing.

Measuring gas transfer in reduced gravity is brutally constrained by time. During a parabolic flight, the aircraft’s steep climbs and dives produce stretches of about 15 to 28 seconds of simulated reduced gravity, far too short for commercial methane sensors, whose response times lag well behind the data window. The team therefore focused on oxygen transfer into an abiotic medium of deionized water held at 37 degrees Celsius, the same temperature as the bacterial cultures the system is meant to serve. They selected a compact 3M Liqui-Cel hollow fiber module with fibers 330 micrometers in outer diameter and pores of 0.1 micrometers, chosen for its high flux, small internal volume and transparent shell that allowed visual monitoring for condensation. Fast optical oxygen sensors with a measured response time of just 0.725 seconds, roughly 27 times faster than the oxygenation process itself, made it possible to resolve the transfer dynamics within each brief window of weightlessness.

The apparatus was a marvel of compact engineering, organized into three interlocking subsystems. A pneumatic loop alternated between pure nitrogen, used to strip dissolved oxygen down to about one percent of saturation, and pure oxygen, delivered at one liter per minute through digitally controlled flow controllers with the line held at 20 PSI. A hydraulic loop pumped 560 milliliters of deionized water through the membrane at five liters per minute, past redundant oxygen sensors, pressure gauges and a hydraulic accumulator that compensated for water lost by evaporation through the membrane. A thermal loop of about four liters of water, heated by a Peltier device and coupled through a shell-and-tube heat exchanger, held the test liquid at 37 degrees with an accuracy of plus or minus one degree, using the thermal mass of the larger volume to suppress fluctuations. Flow-activated switches protected the system from freezing or boiling if circulation ever stopped, a safeguard that would prove prescient.

During the flight, two operators pressed identical buttons at the call of each parabola, a redundancy in case nausea compromised one of them, triggering a seven-second pulse of oxygen through the membrane’s gas side. A triaxial accelerometer mounted near the aircraft’s center of gravity verified that the simulated environments hit their targets: 0.38 g for Martian gravity, 0.16 g for Lunar gravity and under 0.1 g for microgravity, all within 0.1 g of the intended level during the critical measurement window. The team had planned ten parabolas per gravity level, but a dust containment breach from another experiment ended the flight early, and a more insidious problem emerged during the second set of microgravity parabolas. A bubble that had sat stably in the thermal loop under Martian and Lunar gravity drifted free in weightlessness, lodged in a flow switch and silently shut down the temperature control. The researchers discarded the affected cycles, leaving five valid microgravity measurements, and applied a temperature correction to the remaining data.

When the numbers were crunched, the result was as clean as an experimentalist could hope for. Oxygen mass transfer coefficients averaged between 1.11 and 1.16 times ten to the minus four meters per second across all four gravity conditions, a spread so narrow it borders on statistical indistinguishability. Student’s T-tests comparing terrestrial, Martian, Lunar and microgravity datasets produced p-values of 0.41, 0.48 and 0.85, all comfortably above the 0.05 threshold, meaning no significant difference could be detected between any pair of gravity levels. The measured coefficients also landed within 25 percent of a theoretical estimate based on the Sherwood relation for hollow fiber modules and matched values reported in earlier terrestrial studies, lending further confidence to the measurements. The slightly larger scatter seen in the reduced-gravity datasets was traced not to gravity itself but to the vibration and acceleration fluctuations inherent in parabolic flight, which accelerometer data and Fourier analysis confirmed were far quieter during the terrestrial baseline.

The implications ripple outward well beyond bioreactors. Because membrane contactors transfer gas in both directions, they can not only deliver feed gases but also strip waste products from a reaction, and by Le Chatelier’s principle, removing a product can itself accelerate the underlying chemistry. Membrane contactors already maximize biomethane recovery in anaerobic wastewater treatment on Earth, and they are being integrated into electrolyzer electrodes, where gas bubbles that stubbornly cling to surfaces in reduced gravity would otherwise choke off hydrogen and oxygen production. The new result provides a crucial assurance for those designs: if a membrane-enhanced electrolyzer or bioreactor performs differently in space, the credit or blame belongs to the electrochemistry or the microbes, not to the gas delivery hardware. The findings also raise the Technology Readiness Level of membrane gas transfer for space life support, a milestone that agencies weigh heavily when deciding which technologies earn a berth on future missions.

The team is now looking toward the next logical step: a small demonstration membrane-aerated bioreactor designed for long-duration operation aboard the International Space Station, where the biological half of the methane-to-PHB process can finally be tested under sustained weightlessness. In the meantime, the parabolic flight rig itself may prove as valuable as the result it produced. The authors note that the system, capable of completing a full oxygen transfer measurement in under ten seconds through an innovative dual water loop design, could be readily adapted to compare different membrane modules or to measure the transfer rate of any pure gas for which an inline sensor exists, positioning it as a high-throughput characterization tool for next-generation membrane materials. For a field in which every launch opportunity is scarce and every kilogram of hardware is fought over, a technology that works exactly the same on Earth as it does on the Moon, Mars or in the void between them is precisely the kind of boring reliability that space exploration depends on.

Subject of Research: Membrane gas–liquid oxygen mass transfer under variable gravity conditions during parabolic flights

Article Title: Experimental system for investigating membrane gas–liquid mass transfer during a parabolic flight

Article References: Bray, A., Lampe, J., Schwiebert, A. M., Bush, J. A., Jacobs, A., Force, N., Pieja, A., Munakata-Marr, J., & Vanneste, J. (2026). Experimental system for investigating membrane gas–liquid mass transfer during a parabolic flight. Space and Planetary Resources, 2(1), Article 2. https://doi.org/10.1007/s44461-026-00007-w

Image Credits: AI Generated

DOI: 10.1007/s44461-026-00007-w

Keywords: membrane contactor, hollow fiber membrane, gas-liquid mass transfer, parabolic flight, microgravity, Martian gravity, Lunar gravity, oxygen transfer, bioreactor, in-situ resource utilization, space life support, PHB biopolymer